1) Full = full mix, obviously, haha.2) Rhythm = percussion instruments (drums, auxiliary percussion, etc), if you have a driving percussive melodic instrument, like say an electric guitar playing palm-muted 8th notes, you could include that as well. You should ask the company though. But for me, when I'm asked for a rhythm track, I usually only send percussion instruments.3) No percussion = mute all drum percussion tracks.4) Alternate mix = take some elements out so the track isn't so busy. Some libraries call this a "lite mix" or "minus mix" or "narrative mix" or "stripped mix". Just find the distracting stuff like lead melodies, busy percussion, SFX, etc, and mute them all so you have a less busy mix. But definitely DON'T do an entirely new mix to make a different track...... just mute stuff and re-bounce.

Congrats, Jolene!
As for the stems, what VanderBoegh said. When thinking about a lite mix, I always think about removing elements that would be less intrusive underneath softer dialogue. So, if you have lines doubled in octaves, or any kind of ear candy that's asking for too much attention, etc., try toning it down that way.